This invention relates to oxygen sensor for detecting oxygen concentration in a gas mixture such as exhaust gas of an internal combustion engine. The sensor is of the concentration cell type comprising an ion conductive solid electrolyte layer provided with an electrode layer on each side, and the invention particularly relates to the electrode layer.
In an internal combustion engine, the air to fuel ratio of an air-fuel mixture consumed in the combustion chambers of the engine significantly affects both the efficiencies of the engine and the composition of the exhaust gas. Accordingly some of recent internal combustion engines, particularly automotive engines, are provided with a feedback control system for precisely controlling the air/fuel ratio of a combustible mixture fed to the engine based on a certain characteristic of the exhaust gas. In such a control system, the concentration of oxygen in the exhaust gas is usually taken as an indication of an actual air/fuel ratio of the combustible mixture and is detected with an oxygen sensor.
A typical oxygen sensor now in practical use is fundamentally an oxygen concentration cell which consists essentially of a layer of a solid electrolyte whose conductivity is predominantly attributable to the migration of oxygen ions and two electrode layers formed respectively on the front and rear surfaces of the electrolyte layer. When a gas mixture such as an engine exhaust gas is present on one side of the cell and a reference gas such as air on the other side, the electromotive force of the cell depends on the ratio of the oxygen partial pressure of the reference gas to that of the gas mixture.
Various oxide ceramics exemplified by ZrO.sub.2, ThO.sub.2 and Bi.sub.2 O.sub.3 are known as solid oxygen-ion electrolyte materials. These oxides are usually used in the form of a solid solution with a stabilizing oxide such as e.g., CaO, Y.sub.2 O.sub.3 or Nb.sub.2 O.sub.5. Typical examples of commonly used solid solution systems are ZrO.sub.2 --CaO, ZrO.sub.2 --Y.sub.2 O.sub.3, Bi.sub.2 O.sub.3 --Nb.sub.2 O.sub.5 and Bi.sub.2 O.sub.3 --Y.sub.2 O.sub.3. The stabilizing oxide is usually contained in an amount of from about 5 to about 20 mol%.
The electrode layers are formed individually as a microporous coating which is in intimate contact with the surface of the electrolyte layer. The two electrode layers, which are made of either the same material or different ones, must be electronically conductive to serve as charge collectors. Besides, the electrode layers must be physically and thermochemically stable at elevated temperatures since oxygen sensors of the described type are in many cases used in a high temperature atmosphere ranging from about 500.degree. C. to about 1400.degree. C. By reason of these fundamental requirements, a noble metal, particularly platinum, is commonly used as the material, at least as the conductive component of the material, of the electrode layers.
When an oxygen sensor of the described type is used for measuring the oxygen concentration in a gas mixture which contains oxygen and oxidizable gases and is in a non-equilibrated state as is typified by exhaust gas of an internal combustion engine, there arises a problem that a platinum electrode layer of the sensor catalyzes oxidation reactions between oxygen gas and the oxidizable gases contained in the gas mixture. As the result, the oxygen concentration in the gas mixture is measured nearly, if not exactly, in an equilibrated state, and hence the output of the oxygen sensor fails to indicate an actual oxygen concentration in the gas mixture in the original or non-equilibrated state. In connection with a catalytic action of the platinum electrode layer, the described oxygen sensor exhibits a unique output characteristic when the sensor is used in exhaust gas of an internal combustion engine to estimate the air/fuel ratio of a combustible mixture consumed in the engine from the output voltage of the sensor. The output voltage is on a relatively high level when the air/fuel ratio is below a stoichiometric ratio but on a clearly different and very low level when the air/fuel ratio is above the stoichiometric ratio. A great and sharp transition of the output voltage from the high level to the low level, or vice versa, occurs when the air/fuel ratio is around the stoichiometric ratio, so that it is very easy to judge whether the air/fuel ratio is above or below the stoichiometric ratio. However, the output voltages exhibits only a very small change so long as the air/fuel ratio remains on one side, either a higher side or a lower side, of the stoichiometric ratio. Accordingly, it is very difficult to numerically estimate the air/fuel ratio from the output voltage of the sensor exposed to the exhaust gas when the air/fuel ratio is deviated from the stoichiometric ratio. The use of costly platinum is an additional disadvantage of the above described oxygen sensor.
It has been tried to use a metal which is not a noble metal and exhibits no catalytic action on the oxidation reactions of, for example, hydrocarbons and carbon monoxide as the material of the electrode layers of the oxygen sensor. However, such a metal readily reacts with oxygen contained in a gas mixture to be measured at high temperatures. The original non-equilibrated stage of the gas mixture, therefore, is no longer retained when the gas mixture comes into contact with the surface of the electrolyte layer, and accordingly the output voltage of the sensor does not accurately correspond to the original or real oxygen concentration in the gas mixture.
It is an object of the present invention to provide an improved oxygen sensor which is identical with a conventional oxygen sensor of the concentration cell type in its solid electrolyte layer of a stable and noncatalytic material formed on one side of the electrolyte layer to be exposed to a gas mixture subject to measurement.
It is another object of the invention to provide an oxygen sensor of the concentration cell type whose output voltage upon exposure to exhaust gas of an internal combustion engine has a close relation to the air/fuel ratio of a combustible mixture consumed in the engine over a wide range of the air/fuel ratio.
An oxygen sensor according to the inventon has a layer of a solid oxygen-ion electrolyte typified by a stabilized zirconia, a first electrode layer formed on one side of the electrolyte layer to be exposed to a gas subject to measurement and a second electrode layer formed on the opposite side of the electrolyte layer to be exposed to a reference gas. Both the first and second electrode layers are permeable to gas. The sensor is of a known construction in these respects. According to the invention, the first electrode layer is made of a material comprising carbon silicide as a sole conductive component thereof.
The material of the first electrode material may consist of carbon silicide alone but may alternatively contain a minor amount of, usually in the range from about 1 to about 10 Wt% of carbon silicide, trisilicon tetranitride.
The second electrode layer is made of either the same material as the first electrode layer or a commonly used material such as, e.g., platinum.